The present invention relates in general to a method of and an apparatus for acoustic well-logging and in particular it relates to a novel method for determining permeability of a formation insitu.
It is well-known that measurements of the conductivity and porosity of a formation are important in determining whether hydrocarbons are located therein, the hydrocarbon being found as non-conductive fluids in porous rock formations. It is also well-known, that hydrocarbons are generally not extractable from porous rock formations unless those formations are also permeable. At the present time, no apparatus is known for accurately measuring the permeability of a rock formation insitu. Accordingly, after hydrocarbons are detected, it has been generally necessary to obtain a side-wall core of a given formation in order to measure permeability in the laboratory. Such a technique is time consuming and unduly expensive.
Techniques of acoustic well-logging are also well-known and the possible applicability of such techniques to the determination of the permeability of a formation was predicted by Rosenbaum in the article "Synthetic Microseismograms: Logging In Porous Formations", Geophysics, Volume 39 Number 1 (February, 1974).
Rosenbaum investigated numerically, the ideal case of a borehole filled with an inviscid fluid surrounded by a formation that is porous and which conforms to Biot's theory. His investigation showed that the effect of the pore-fluid mobility on the calculated response is large and can be measured with an appropriate logging tool. He further predicted that the relative amplitude of a tube wave obtained from a sonic tool with a wide band frequency response would depend upon the permeability of the formation.
Reacting to the predictions of Rosenbaum, at least one attempt has been made to employ standard sonic logging tools for the determination of permeability. In a paper entitled "Permeability Profiles From Acoustic Logging", by J. J. Staal and J. D. Robinson, presented to the 52nd Annual Fall Technical Conference and Exhibition on the Society of Petroleum Engineers of A.I.M.I. held in Denver, Colo., Oct. 9-12, 1977, it was reported that with a standard Schlumberger sonic probe having transmitter-receiver spacing of up to 5 feet, a correlation could be detected between permeability and tube wave attenuation.
While standard sonic logging tools might be applicable for permeability determination, a need exists for improved techniques for permeability determination.